1. Field of the Invention
The present invention relates to laser rods. More specifically, the present invention relates to thermal birefringence compensated laser rods.
2. Description of the Related Art
The beam quality and output power of a laser oscillator are severely degraded by as little as a quarter wave of thermal birefringence. The source of thermal birefringence is side cooling of a pumped cylindrical laser rod, which results in a radial temperature gradient within the rod. This is a consequence of heat conduction, where a thermal gradient is necessary for heat transfer. Solving the heat conduction equation in cylindrical coordinates shows that the temperature profile in a long, isotropic, uniformly pumped laser rod is a quadratic function of radius. Both the index of refraction and the physical length of the rod change with temperature, so that the rod becomes a thermally induced lens. The rod becomes birefringent due to differential stress in the radial and tangential directions (circular symmetry), depending on such factors as: (a) the material the rod is made from; (b) the dimensions of the rod; and (c) the heat loading of the rod. The net effect is that phase shifts for radial and tangential polarizations are different, even though both are quadratic functions of the radius, just as in a spherical lens.
Scott-Dewit compensation is a well-known method by which the thermal birefringence of a rod can be corrected, either by another rod or by the same rod. However, Scott-Dewit compensation is difficult on a chain of different diameter amplifiers because the birefringence of one amplifier rod is used to compensate another. For a single rod double pass amplifier, Scott-Dewit compensation works fairly well, but requires the use of a Faraday rotator. Vector phase conjugation works extremely well, but is complicated to implement.
Other inventions by this Applicant also assigned to Raytheon Company include: xe2x80x9cReeder Rotatorxe2x80x9d (Ser. No. 09/483,254, U.S. Pat. No. 6,268,962; xe2x80x9cReeder Compensatorxe2x80x9d (Ser. No. 09/482,376, U.S. Pat. No. 6,317,450; and xe2x80x9cWaveplate Polarizing Rotatorxe2x80x9d (Ser. No. 09/482,378; These applications are incorporated herein by reference.
What is needed then is a mechanism for compensation of thermal birefringence effects in laser rods to improve the performance of high average power laser oscillators and amplifiers. More specifically, what is needed is a mechanism that virtually eliminates the birefringence problem for laser pump powers that would usually produce two or three waves of birefringence. It should be noted that since it takes eight times as much pump power to produce two waves of birefringence as it does to produce a quarter-wave, output power from a thermal birefringence compensated laser oscillator is also on the order of eight times higher. Thus, what is needed is a laser oscillator employing a thermal birefringence compensated laser rod to thereby ensure both good beam quality and greatly increased output power.
The need in the art is addressed by the laser rod of the present invention. The inventive laser rod includes a gain medium and an optical rotator. Preferably, the gain medium includes first and second equal length portions sharing a common optical axis while the optical rotator is disposed between the first and second portions. In the illustrative embodiment, the optical rotator includes a pair of waveplates which receive a polarized beam having a first state and outputs a polarized beam having a second state rotated 90xc2x0 with respect to the first state.
Additionally, a laser apparatus providing optical gain while permitting thermal birefringence correction includes at least one optical rotator disposed between a pair of equal length optical gain elements. Preferably, the optical rotator includes a first waveplate receiving a polarized beam having a first state and generating the polarized beam having a second state, and a second waveplate oriented 45xc2x0 from the first waveplate, which receives the polarized beam at the second state and produces the polarized beam having a third state, wherein the first and third states differ from one another by 90xc2x0 Advantageously, multiple rotators can be employed to compensate strongly birefringent rods, each rotator compensating one pair of equal length optical gain elements.